Internal combustion engines produce exhaust gases containing a variety of pollutants, including nitrogen oxides (“NOx”), carbon monoxide, and uncombusted hydrocarbons, which are the subject of governmental legislation. Emission control systems are widely utilized to reduce the amount of these pollutants emitted to atmosphere, and typically achieve very high efficiencies once they reach their operating temperature (typically, 200° C. and higher). However, these systems are relatively inefficient below their operating temperature (the “cold start” period).
For instance, current urea based selective catalytic reduction (SCR) applications implemented for meeting Euro 6b emissions require that the temperature at the urea dosing position be above about 180° C. before urea can be dosed and used to convert NOx. NOx conversion below 180° C. is difficult to address using the current systems, and future European and US legislation will stress the low temperature NOx storage and conversion. Currently this is achieved by heating strategies but this has a detrimental effect of CO2 emissions.
As even more stringent national and regional legislation lowers the amount of pollutants that can be emitted from diesel or gasoline engines, reducing emissions during the cold start period is becoming a major challenge. Thus, methods for reducing the level of NOx emitted during cold start condition continue to be explored.
For instance, PCT Intl. Appl. WO 2008/047170 discloses a system wherein NOx from a lean exhaust gas is adsorbed at temperatures below 200° C. and is subsequently thermally desorbed above 200° C. The NOx adsorbent is taught to consist of palladium and a cerium oxide or a mixed oxide or composite oxide containing cerium and at least one other transition metal.
U.S. Appl. Pub. No. 2011/0005200 teaches a catalyst system that simultaneously removes ammonia and enhances net NO conversion by placing an ammonia-selective catalytic reduction (“NH3—SCR”) catalyst formulation downstream of a lean NO trap. The NH3—SCR catalyst is taught to adsorb the ammonia that is generated during the rich pulses in the lean NO trap. The stored ammonia then reacts with the NO emitted from the upstream lean NO trap, which increases NO conversion rate while depleting the stored ammonia.
PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purification system which includes a NO storage catalyst arranged upstream of an SCR catalyst. The NO storage catalyst includes at least one alkali, alkaline earth, or rare earth metal which is coated or activated with at least one platinum group metal (Pt, Pd, Rh, or Ir). A particularly preferred NO storage catalyst is taught to include cerium oxide coated with platinum and additionally platinum as an oxidizing catalyst on a support based on aluminum oxide. EP 1027919 discloses a NO adsorbent material that comprises a porous support material, such as alumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1 wt % precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina is exemplified. U.S. Appl. Pub. No. 2012/0308439 A1 teaches a cold start catalyst that comprises (1) a zeolite catalyst comprising a base metal, a noble metal, and a zeolite, and (2) a supported platinum group metal catalyst comprising one or more platinum group metals and one or more inorganic oxide carriers.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems, particularly under cold start conditions. We have discovered a new passive NOx adsorber that provides enhanced cleaning of the exhaust gases from internal combustion engines. The new passive NOx adsorber also exhibits improved sulfur tolerance.